Scriptwriting is as much a technical exercise as a creative one. I often get hung up on logic and plot details at the planning stage but once I have them down, I can't write the script fast enough. If you feel you have a developed idea and it just needs writing, you're in a very enviable position - many writers won't even get that far.
You have to find a way to fall in love with your project and get excited about writing it. If you don't have that passion during the writing, it won't come across on the page.
You have to identify the power that your original idea had over you and utilise it so that you can steamroll through the bad times and remember why you started in the first place. As I said, the only time I get disillusioned is when I haven't done the planning and get bogged down in the plot. But if it all works, I find actually writing the script is one of the most enjoyable things I can do. It's exhausting and lonely and thankless but so extraordinarily cathartic.
I think it’s because writing is so child-like (I don't mean simple, I mean in the emotional, inner-self way) because it relies so heavily on your imagination. Creating characters and coming up with worlds and situations that not only make for good reading but can elicit an emotional response - it's just like playing with toy soldiers in a sandbox or whatever your equivalent was. They were just little bits of plastic but dammit, didn't you care whether the heroes won and the villains lost? Wasn't it just awful when the little sniper soldier with the bent rifle was captured by the Nazis? And didn't you cheer when your special forces commando unit broke him out and got everyone safely home?
Stop laughing. My point is, you have to find away to get excited about your work - so excited, that it's all you can think about and you can't wait until you have a spare half hour just so you can start getting it down.
It won't be perfect but writing is rewriting. The most important part at the first draft stage is the core emotion of each scene. To that effect, before I start writing, I will often distil each scene down into one word that best describes the emotion I will be writing - regret, envy, anger - and more often than not, those will develop into little chapter titles (like chapters in a book or those chapter titles you get in DVD booklets) so you can break your script down into, I don't know, maybe thirty little sub-headings like, Trouble at the morgue or Killing Mr. Linson - I had one scene, a big end of first act can of worms - simply entitled Oh, sh*t.
You start to think of your work tonally - it becomes something more than technical writing at that point because you're not writing, "that scene where Francis is caught and questioned by the bad guys," you're writing "Francis loses a toe." It's immediately more exciting, more capable of emotion and sounds like much more fun to write.
Get excited again! If you don’t, nobody else will.